In conventional gradient systems, gradient amplitude and speed are limitations in a number of important applications. Examples of MRI techniques where image quality is limited by gradient amplitudes and rise time limits include echoplanar (EPI), turbo spin echo (TSE), steady state free precession (SSFP), and dynamic contrast enhanced 3D gradient echo (DCE MRI) techniques. For EPI, which alternates polarity of the readout gradient to acquire all measurements for an image after a single excitation pulse, the minimum readout time is determined by the maximum gradient slew rate and amplitude. During readout, the magnetization decreases due to T2*decay, resulting in substantial image blurring and distortion. When operating at the gradient limits, increasing resolution requires a longer readout causing increased blurring and image distortion. Thus, gradient performance factors dictate a resolution limit for EPI sequences. Similar blurring problems occur for long echotrain TSE sequences where gradient performance can limit maximum spatial resolution or require shorter echotrains and longer scan times. Acquiring at higher resolutions requires longer echo times (TE) in nearly all conventional sequences, which can cause, at a minimum, a loss in SNR.
Gradient performance is a major limitation in achieving high spatial and temporal resolution in dynamic MRI techniques such as in DCE MRI of the breast. Both contrast-enhanced lesion morphology and temporal pattern of contrast enhancement are useful in the detection and characterization of breast cancer. DCE MRI is typically performed with a 3D gradient-recalled echo (GRE) sequence, with the shortest repetition time (TR) allowed by the gradient capabilities. Faster and stronger gradients allow shorter TR's, leading to reduced susceptibility artifacts and faster imaging. Faster imaging allows higher spatial resolution for better lesion morphology assessment and higher temporal resolution for better characterization of lesion enhancement.